I had the same problem with 3 different pairs of this crapy device.
After troubling around I pressed:
Vol up and down together and after like 3 sec I press the power on button also.
Then suddenly a purple light appeared and I was able to get the thing in pairing mode.
Hope it helps you all. Fact is that this is a stupid device if it was not that it's perfect for running I would never use it again.
Question by Koen 2:22pm, 3rd June 2021